> All the code for embeddable languages I've seen to this point> have been "run till the end of the code" types like icon, tcl,> perlv5, etc.

I wanted to do something similar several months ago and hunted around to
no avail. I ended up writing a threads package (the system I was using
was pretty obscure) and making Tcl sit on top of it (I didn't have to
modify Tcl in any way).

I'd be of the opinion that you might be better off to do more or less what
I did, and sit Python, SCM, or some other embeddable language on top of a
threads package (pthreads would seem to be a good candidate, since
implementations of it are now fairly widely available); I find that this
approach allows me to use any potential concurrency `properly', and saves
the effort of trying to hack together something which gives much the same
effect.